I don't know about the no bump unless lowering price. I know what you're getting at as the threads get out of control with some people bumping incessantly, but at the same time, the forum moves so quick at certain times of the year that FS posts get buried rather quickly.
I like the other ideas though. I would also recommend a stickied thread for those who had good/bad experiences buying on the forum. That way buyers/sellers can search that thread and get some comfort for who they're dealing with.